1. Field of the Invention:
This invention relates to an apparatus for impressing relief patterns of high resolution, particularly phased holograms or phased diffraction gratings, in a thermoplastic information carrier.
2. Description of the Prior Art
It is known to store information in the form of relief patterns of high resolution, such as phased holograms, phased diffraction gratings and the like, on thermoplastic carrier material. The relief pattern is impressed in the thermoplastic carrier by applying pressure and heat. The carrier may be in card or strip form, depending on its purpose. The relief patterns impressed, containing, for example, video information or information about genuineness, can be optically machine-read.
In a known apparatus for impressing phased holograms, a thermoplastic information carrier and an impressing matrix, in the form of an endless strip carrying the hologram relief pattern, are passed together through two calender rollers. One of the two calender rollers is heated, and during its passage between the rollers the thermoplastic film is heated to an appropriate temperature and subsequently separated from the impressing matrix. This arrangement enables the material to pass through at relatively high speed. However, the indirect heating of the impressing matrix by the calender roller has been found to bring various disadvantages. The heat capacity of the heated calender roller is necessarily relatively large, which means that when the impressing apparatus is put into operation the impressing matrix only reaches the desired temperature after a fairly long heating-up time. For example, there may be long periods of waiting when work has been interrupted, and a high reject rate may sometimes have to be accepted in the starting-up phase. Similarly, the impressing temperature can only be changed slowly, for example, when changing to a different carrier material. Moreover, a large amount of heat energy is required, particularly as the impressing matrix is heated over its entire width, although in most cases only a relatively narrow strip in the middle of the impressing matrix carries a relief pattern.